1. Field of the Invention
The invention relates generally to superconductive integrated circuits, and more particularly with respect to Josephson junction integrated circuits of the magnetically controlled interferometer type.
2. Description of the Prior Art
A Josephson junction comprises two superconductive layers with a thin barrier layer interposed therebetween, which, when in a superconductive state, may permit superconductive electron pairs to tunnel therethrough. It is a peculiar characteristic of the Josephson junction that it can exist in two distinct states, the zero voltage state and the nonzero voltage state. The nonzero voltage state is attained by exceeding the current carrying capability of the zero voltage state, the required current being called the critical current of the junction and the critical current being a property of the junction. A further property of the critical current is that it may be modulated by the application of a magnetic field parallel to the plane of the thin barrier layer.
The Josephson junction has been employed as a logic gate, using either a control current exceeding the critical current, or a predetermined magnetic field, usually applied by means of passing a current thru a nearby conductor to effect the switch from zero to nonzero voltage. In the practice of the art of Josephson circuit fabrication, the single junction has been replaced by logic gates consisting of two or more junctions, along with conventional circuit elements, such as resistors, inductors and capacitors. The reason for this involve fundamental issues such as current gain, isolation and scaling behavior, and are well known in the art. Josephson logic gates further subdivide into two basic types. Current injection gates are switched by the application of control current directly into the gate, in analogy with current switching in a single junction. Josephson current injection logic gates are of relatively simple construction, but since the control current is applied in common with a bias current to the gate, isolation between the input and output components is not complete.
A second type of Josephson logic gate is the magnetically controlled gate, commonly known as an interferometer or squid (for superconducting quantum interference device), the former term being preferred by those practicing the art of Josephson integrated circuit fabrication. In the interferometer, two (or more) junctions are placed in parallel with the bias current and in series with a loop inductor therebetween. A control inductor is coupled by a mutual inductance to the loop inductor. Passage of control current thru the control inductor, or control line, causes an induced current to flow in the loop containing the two junctions and the loop inductor. This current adds to the bias current in one junction in the loop and substracts from the bias current of the other junction. If the additive currents exceed the critical current of that junction, it switches to the nonzero voltage state, thereby shunting the full bias current to the otherjunction and interrupting the flow of loop current. For properly chosen parameter values, the second junction also switches to the nonzero voltage state, thereby completing the switching of the interferometer as a whole, and shunting the bias current into an output transmission line for further use in controlling subsequent logic gates. Note that control of an interferometer is by means of magnetically coupling to the loop inductor, whereas magnetic control of a single junction is effected by coupling to the barrier layer of the junction itself. The magnetically controlled logic gate permits isolating the bias current from the signal current but has the disadvantage of requiring a larger number of structural layers and therefore, a larger number of manufacturing steps. In addition, since many of the required patterns are made by evaporation or sputtering of thin films defined by etching or photoresist lift-off, the large number of manufacturing operations often results in a low device yield.
In U.S. Pat. No. 4,430,662, issued Feb. 7, 1984 and assigned to the assignee of the present invention, filed Apr. 9, 1981, by Don W. Jillie, Jr. and Lawrence N. Smith, assigned to the Assignee of the present application, a technique was described for combining the functions of the Josephson lower electrode and ground plane into one superconductive layer, thereby eliminating one superconductive layer and one insulator layer from the then state of the art fabrication process as applied to current injection logic gates. It is an object of the present invention to show how to construct Josphson magnetically controlled logic gate interferometers using the fabrication sequence developed for current injection logic gates, thereby eliminating two layers from the otherwise more complex structure. Beneficially, the process sequence and component layout described in the present invention requires only five layers and only five photo-resist patterning steps plus a barrier and an anodization step for the Selective Niobium Anodization Process (SNAP), which is fewer than other means of fabricating these gates. The SNAP, as described in U.S. Pat. No. 4,421,785 issued Dec. 20, 1983, in the name of Harry Kroger, and assigned to the Assignee of the present invention, requires at least seven depositing and patterning steps to fabricate a conventional interferometer. A magnetically controlled logic gate constructed by conventional prior art procedures, such as with lead alloy superconductive layers, requires as many as 12 layers and 12 patterning steps, plus a junction barrier formation step.